Empirical bond additivity scheme for the calculation of enthalpies of vaporisation of organic liquids
The enthalpy of vaporisation ΔHv° of an organic liquid can be expressed empirically as the sum of additive bond terms from which must be subtracted an amount proportional to the sum of the differences in group electronegativities between groups linked by carbon–carbon bonds. This latter term is deemed to arise from disruption of carbon–carbon bonding compared with the free molecule, consequent upon the formation of intermolecular bond–bond interactions. For C4–C8 alkanes, experimental values of ΔHv° can be reproduced with a mean discrepancy of ca. 0.3 kJ mol-1, with the introduction of only two new adjustable parameters. For halogenoalkanes, alkanols, ethers, thiols, thioethers and amines, an additional term is necessary for C–H bonds α to the functional group; this can be partially explained in terms of bond polarities, obtained by electronegativity equilibration. For alkanes and alkane derivatives, including nitriles, aldehydes and ketones, the C–C bonds apparently make no contribution to the enthalpy of vaporisation. Compared with earlier empirical schemes, the agreement between experimental and calculated values is just as satisfactory, but with fewer adjustable parameters.